The average rate is the total change in a measured quantity divided by the total time taken over a period of the reaction.
The average rate of reaction is the total change in a measured quantity divided by the total time taken over a period of the reaction. Malay: kadar purata · Chinese: 平均速率.
An average rate smooths out a whole time interval into a single value. You take how much a quantity changed, such as the volume of gas produced or the mass lost, and divide it by how long that change took. On a volume-time graph, the average rate between two points is the gradient of the straight line joining them, not the slope of the curve itself.
Example. If a reaction releases 60 cm3 of gas in the first 30 seconds, the average rate over those 30 seconds is 60 divided by 30, which is 2 cm3 s−1. If the same reaction then produces only 15 cm3 more gas in the next 30 seconds, the average rate has fallen, because the reactants are running low and the reaction is slowing down.
Confusion to avoid. Do not confuse the average rate with the instantaneous rate. The average rate covers a whole interval and is usually easy to calculate from two readings, while the instantaneous rate is the rate at one exact moment, found from the gradient of a tangent to the curve. Early in a reaction the instantaneous rate is higher than the average rate, because the reaction starts fast and then slows.
The average rate links straight to the concept of rate that opens this chapter, and it is the quantity most SPM calculation questions ask you to work out from a table or a graph, so practise reading the correct start and end values for SPM Chemistry.
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